I. Field of the Invention
This invention relates to a fusing apparatus as is commonly used in xerographic copying machines, and more particularly to a cold pressure fusing apparatus for fusing toner images on a support surface, such as a sheet of paper, by applying a plurality of pressure fusing roller strokes to a toned copy sheet.
II. Description of the Prior Art
In a typical xerographic process a photoconductor comprising a photoconductive composition coated on a rigid or flexible substrate is uniformly electrostatically charged in the dark and then exposed by being illuminated in an image pattern in accordance with graphic material on an original document. The photoconductor becomes discharged in the areas exposed to the illumination, but retains its electrostatic charge in the dark areas, which areas correspond to the graphic material on the original document. The resulting electrostatic latent image is developed by depositing on the photoconductor a finely divided electrostatically attractable developing material (toner). The toner will normally be attracted to those areas on the photoconductor which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This visible image of developing material is then transferred to a support surface, such as plain paper or any other suitable substrate, to become the ultimate copy. Any residual developing material remaining on the photoconductor is removed and the photoconductor is reused as described above for subsequent copies. The toner image that was transferred to the plain paper is then fixed thereto. This can be accomplished by using a developing material that is heat fusible with the application of sufficient heat and pressure to the paper to cause the developing material to melt and be fused into the paper so as to be permanently affixed thereto, or by using a cold pressure fusing technique where, for example, cold pressure fusible toner particles on a paper substrate are passed under pressure through the nip formed between a pair of opposed and cooperating hard surfaced rollers whereby the toner particles are by pressure alone fused into the paper.
Typically, in the so-called heat pressure fusing apparatus, the paper with the toner image thereon is passed between a pair of opposed and cooperating rollers, at least one of which is heated. Generally, the heated roll is formed of a hollow cylinder having a radiant heater, such as an infrared lamp or a halogen lamp, centrally located within the cylinder to heat the roll, in series with a bimetal thermostat. During operation of the fusing apparatus, the paper to which the toner images are electrostatically adhered, is passed through the nip formed between the rolls with the toner image contacting the fuser roll to effect heating of the toner image within the nip. Fusing is enhanced by the second roll or pressure roll as it is commonly called as the result of a biasing force which forces the rolls into engagement. This type of fusing apparatus, while exhibiting effective results in many instances, does have certain disadvantages. For example, heat fusing requires that a rather large supply of heat be available. This typically requires a supply of electric power in large quantity which is expensive. Furthermore, the amount of heat generally used in a heat fusing apparatus present the possibility of overheating the substrate as well as the toner, thereby producing some danger of a fire.
To avoid the disadvantages of the heat and pressure fusing systems, it is preferred in many instances to use a cold pressure fusing system where electrostatic toner powder images on a substrate are fixed to the substrate by the application of pressure alone. However, cold pressure fusing systems also present some difficulties. For example, since the fixing of the toner to the substrate is accomplished by pressure alone, large amounts of pressure must be applied to the rollers which therefore must of necessity be constructed of sufficient strength and size to withstand these pressures. This, of course, tends to make these systems mechanically complex, inefficient, and expensive. Employing large amounts of pressure to a toned copy sheet also tends to interfere with the quality appearance of the fused image. Examples of some prior art systems which have sought to overcome some of the above-described difficulties are described in U.S. Pat. Nos. 3,931,793 and 3,988,061 and British Pat. No. 2,006,111.
The present invention seeks to overcome the basic problems associated with cold pressure fusing, and provide a cold pressure fusing system which is efficient, employs lower fusing pressures, and a system that will tend to improve the appearance of the final copy.